So, if you’re completely at a loss as to how to start… You’re not ready to start (on the project you imagine). You’re going to have to start with some smaller goals.
Do you have ZERO experience with building web stuff? Or zero experience with coding/scripting/programming/drag-n-drop designing anything? If that’s the case, I’d recommend doing some more advanced tutorials such as the one that (I think) @dan1 has that gets you building a grocery list or to-do list type of app. (Also, since you’ve done the lessons, do them again in “hard mode.”)
Doing stuff like that will give you insights into very basic (but very practical) app design principles and what I call “doing basic web stuff.”
You might also read up on MVC (“Model - View - Controller”) apps which is exactly the metaphor that Bubble uses. The Database tab is “Model”, the Design tab is “View”, the Workflow tab (and Conditional tabs on elements in Design view, BTW) is the “Controller.” But all of that is sort of theoretical. If you already have a sense for how web apps work (but just never had the chops to build one yourself), you’ll find that Bubble has a ton of thoughtful features designed to get you going very quickly.
(Many of the questions here in the forum are about running into limitations of those thoughtful features and the answer is, invariably, “well, smarty pants, now you need to go learn how you REALLY do that.”)
If you DO have web design experience (e.g., you’ve built simple or even advanced single-user brochure-ware type sites using any sort of tool – be it a desktop tool or online services like Squarespace or whatever) you should be pretty darn excited by Bubble as it lets you do anything you could do with those static/single-user web design tools, but make a multi-user app-type version of that. And you’d already have a sense for how that might work.
It’s the multi-user / built-in database stuff in Bubble that’s the exciting part. Everything you build is magically multi-user and magically provisioned – it’s zero deploy time, right? Everything(ish) you need is already right there.
But if you have no idea how to get started, you need to see and do some practical examples of things to develop a bit of an innate sense for how web applications work. There may be good books about this. There may be decent sites about this.
But, mostly, I’d recommend learning by doing. There are some things you can get started on right away. For example, you want your site to have users who are buyers and sellers of… something. Ok, so you want user profile pages. That’s easy. Go build that.
Start a new Bubble project. Start with the template (this gives you the nice login/signup stuff all built into a friendly header that saves you a lot of time). Now go build out what happens when a User signs up for the site. Just make it a really simple onboarding flow. Collect the User’s name and email and a profile picture and some other relevant fact.
Get to the point where you can log in to your own site (you’re going to need this anyway). Now go build a profile page. Display some info about the User. Make a page that lets you GET to the profile page of a User.
You’ll learn a metric s-ton of stuff in trying this. (And, again, you will have to know how to do all of this anyway to achieve your production app idea.)
So, just go build a completely pointless app. It does nothing but lets people sign up and see other people that have signed up. There’s no functionality, only framework. So that’s a good place to start while you learn other things. At the end of that you’ll also have an idea for how you might build out product pages and the actual GUTS of the usable part of your site.
The good news is, once you’re cool with all that other stuff, adding subscriptions and payments and such is just incredibly simple.
I’m fairly experienced with programming all sorts of things (by which I mean not “coding web apps”, but algorithmic thinking and hacking all sorts of things), and what I started with in Bubble is tackling the multi-user thing (which, again, is one of the greatest things in Bubble as it’s mostly just automagic for you and, to my mind, the thing I’d never want to bother with building from scratch).